1. Field of the Invention:
The present invention relates to a wavesource image visualization method based on hologram measurement data obtained by radio wave hologram observation.
2. Description of the Related Art:
Radiowave visualization methods that employ wave interference include both active visualizing techniques such as in synthetic aperture radar, snow radar, and underground radar as well as passive visualizing methods such as radioastronomy and radiometry [(1) Masaru Matsuo, Kuniyoshi Yamane: Radar Holography, Electronic Communication Society, Ed., 1980. (2) Yoshinao Aoki: Wave Signal Processing, Morikita Shuppan, 1986.]. Such methods have already been proposed for applications in probing wavesources of unwanted radiowave emission [(3) Jun'lichi Kikuchi, Motoyuki Sato, Yoji Nagasawa, Risaburo Sato; A Proposal for Searching the Electromagnetic Wave Sources by Using a Synthetic Aperture Technique, IECE transactions on B, Vol. J68-B, No. 10, pp. 1194-1201, October 1985.]. Spectral high-resolution algorithms by means of MLM (Maximum Likelihood Method), MEM (Maximum Entropy Method), and MUSIC (MUltiple SIgnal Classification) [(4) S. Kesler, Ed.: Modem Spectrum Analysis II, IEEE Press, 1986)] or improved ESPRIT (Estimation of Signal Parameters via Rotational Invariance Techniques) [(5) R. Roy and T. Kailath: ESPRIT-Estimation of Signal Parameters via Rotational Invariance Techniques, IEEE Transactions on Acoustics, Speech, and Signal Processing. Vol. ASSP-37, No. 7, pp. 984-995, July 1989.] have also been proposed for realizing higher resolutions in the probing of a distant wavesource. In addition, examples of the application of MEM algorithms to the probing of a nearby wavesource have also been reported [(2) Yoshinao Aoki: Wave Signal Processing, Morikita Shuppan, 1986. (6) Jun'ichi Kikuchi, Yoji Nagasawa: Search for Electromagnetic Wave Sources by Using the Maximum Entropy Method, IECE Transactions on B, Vol. J69-B, No. 9, pp. 949-957, September 1986.].
MLM, MUSIC, and ESPRIT, however, are limited to the estimation of point sources, and in MEM, either a point wavesource reconstruction mode or a wavesource reconstruction mode having a normal distribution spread can be selected in accordance with the number of prediction error filter terms.
When dealing with unwanted emission from an actual device, however, the wavesource is rarely distributed as a limited number of point-like sources and is more likely to be an expansive planar wavesource. In other words, in visualizing the wavesource distribution of an unclear configuration, the above-mentioned algorithms are considered inappropriate. Visualization that takes unwanted emission as its object should therefore take the wavesource distribution as a continuous wavesource distribution rather than as an assemblage of a limited number of point wavesources. The inventors of the present invention have also proposed an algorithm SPIM (Spectrum Phase Interpolation Method) and its application for reconstructing a correct wavesource image of a configuration in which point wavesources coexist with a wavesource that spreads over an area [(7) Hitoshi Kitayoshi: High Resolution Technique for Short Time Frequency Spectrum Analysis, IEICE Transactions on A, Vol. J76-A, No. 1, pp. 78-81, January 1993; (8) Hitoshi Kitayoshi: High Resolution Technique for 2-D Complex Spectrum Analysis, IEICE Transactions on A, Vol. J76-A, No. 4, pp. 687-689, April 1993].
Nevertheless, this method also requires the setting of a threshold value for estimating a point wavesource and is therefore inadequate for the visualization of unwanted emission. An example has been reported in which, as a visualization technique for dealing with a continuous wavesource distribution, the relation between the observed field and a wavesource of an arbitrary shape is represented as a Fresnel-Kirchhoff diffraction integral [(9) Yoshinao Aoki, Shigeki Ishizuka: Numerical Two-Dimensional Fresnel Transform Methods, IECE Transactions on B, Vol. J57-B, No. 8, pp. 511-518, August 1974]. This method, however, imposes a limit on the region that can be visualized because it takes a Fresnel region as its object. This method further suffers from the drawback of bad resolution at the edges of the visualized range because a quasi-Fourier transform is used and the reconstruction results become wave space.
Finally, research into field measurement that takes into consideration the directivity of the antenna includes field measurement in the vicinity of the antenna [(10) A.D. Yaghjian: An Overview of Near-Field Antenna Measurements, IEEE Transactions on Antennas and Propagation, Vol. AP-34, No. 1, pp. 30-45, January 1986; and (11) Tasuku Teshirogi, Near-Field Antenna Measurement, IECE Magazine, Vol. 62, No. 10, pp. 1145-1153, October 1979.]. In these cases, however, field distribution is measured using a sufficiently large measurement surface (D.times.D) with respect to the antenna (L.times.L), which is the object of observation, under conditions in which L&lt;D. The probe antenna used in measurement, considered in terms of its characteristics, is a device for investigating far-field, and the method therefore cannot be used unaltered under conditions in which L&gt;D.
Next will be explained a radio hologram numerical reconstruction method of the prior art that does not employ Fresnel approximation or Fraunhofer approximation.
A surface current source J(R') exists at wavesource point x', y', z' on the orthogonal coordinates shown in the hologram observation model of FIG. 1, and the field of observation point x, y; z=0 thereby created is E(R). In addition, the range of distribution of wavesource J(R') is a finite two-dimensional plane at z=z' where:
-L/2.ltoreq.x'.ltoreq.L/2 and -L/2.ltoreq.y'.ltoreq.L/2; PA1 -D/2.ltoreq.x D/2 and -D/2.ltoreq.y.ltoreq.D/2. PA1 a step of segmenting the reconstruction region of a hologram reconstructed image by the size of the hologram measurement surface; PA1 a step of executing a first FFT (Fast Fourier Transform) after multiplying hologram measurement data by a weighting function; PA1 a step of executing a second FFT after giving the reconstruction focal distance and a numeral pair designating a segment of a reconstructed image, and multiplying by a weighting function a value obtained by multiplying an antenna sensitivity inverse matrix for each segment by an inverse propagation function; PA1 a step of executing IFFT (Inverse Fast Fourier Transform) after multiplying the output of the first FFT by the second FFT; PA1 a step of compensating, in segment units, the weighting function used in multiplying the hologram measurement data; PA1 a step of compensating, in units of the reconstruction region, the weighting function used in multiplying the value obtained by multiplying the antenna sensitivity inverse matrix for each segment by the inverse propagation function; PA1 a step of displaying the output of this step at a position that designates the segment and making the output of that step a segment reconstructed image; and PA1 performing all of these operations for all segments, and producing a hologram reconstructed image. PA1 a step of carrying out radio hologram observation on two scan surfaces, a first scan surface and a second scan surface, placing at least two probe antennas, a horizontal polarization antenna and a vertical polarization antenna, on each scan surface, and recording the voltage vector distribution received by the antennas; PA1 a step of calculating the total directional characteristics of the probe antennas using a moment method, and finding the reception antenna sensitivity matrix of the horizontal polarization reception antenna and the reception antenna sensitivity matrix of the vertical polarization reception antenna due to wavesource current vectors; and PA1 a step of comparing the determinants of the reception antenna sensitivity matrices, finding the magnitude of error of the reception antenna sensitivity matrix of the vertical polarization reception antenna and error of the reception antenna sensitivity matrix of the horizontal polarization reception antenna, and selectively using received voltage vectors and reception antenna sensitivity inverse matrices to find and visualize three wavesource current vector components.
and the range of observation of E(R) is also a finite two-dimensional plane at z=0 where:
Here, using a dyadic Green's function G in three-dimensional free space, E(R) can be represented by: ##EQU1##
However, if the effective vector length of an antenna used for observing field vector E is set at l.sub.1 and the wavelength is .lambda., the voltage V for receiving in the region r=.vertline.R-R'.vertline.&gt;&gt;.lambda. is: EQU V=g l.sub.1 .multidot.E (2)
Here, g is a constant. In other words, l.sub.1 can be seen as a function indicating directivity and sensitivity that does not depend on distance r. From equations (1) and (2), the output voltage from an antenna positioned at observation point R can be given by: ##EQU2##
Equation (3) gives reception voltage by any antenna with respect to any surface current distribution. If the antenna scan system is considered in actual hologram measurement, however, an antenna that receives the field in the x and y directions can be easily realized, but accurate reception of the field in the z direction is extremely difficult. Despite the presence of a current vector of the z component on the surface of the device which is the object of measurement, almost nothing is received in the hologram measurement plane.
The discussion hereinbelow is limited to a case in which the plane current has only a horizontal component and a vertical component, and reception is by antennas of horizontal polarization and vertical polarization.
Antennas of horizontal polarization and vertical polarization are placed in the hologram measurement plane, and the voltage vectors thereby received are: ##EQU3##
Here, V.sub.h and V.sub.v are the received voltages of the horizontal polarization and vertical polarization antennas, and l.sup.h.sub.1 and l.sup.v.sub.1 are the effective vector lengths of these antennas. The device employs two reception antennas of the same form, the main polarization sensitivity is A.theta., and the cross polarization sensitivity is A.phi.. In addition, the zenith angle and azimuth angle of point R' are .theta..sub.h and .phi..sub.h as seen from horizontal polarization reception antenna positioned at point R, and the zenith angle and azimuth angle of point R' are .theta..sub.v and .phi..sub.v as seen from the vertical polarization reception antenna. Here, azimuth angles .phi..sub.h and .phi..sub.v represent angles measured with respect to the direction of the x axis and y axis, respectively.
If the portion included on the right side of equation (3) that represents the reception sensitivity due to the point current source is represented as: ##EQU4## V.sub.h and V.sub.v can be found from the following equation, based on equations (3) and (4): ##EQU5##
On the other hand, equations (5) and (6) produce: EQU J(R')=A.sup.-1 (R-R').multidot.V(R).multidot.exp(j2.pi.r/.lambda.).multidot.rdxdy(7)
According to equation (4), the received voltage vectors are: ##EQU6##
And from equation (7), only two wavesource current vector components are visualized for these voltage vectors: ##EQU7##
The above-described prior art has the following problems:
Based on equation (6), V(R) has NxN items of data and visualized point of J(R') has MxM items, thereby requiring computation on the order of N.sup.2 .times.M.sup.2 and further requiring a considerable amount of time to obtain the visualization results.
Wavesource current vector cannot be accurately measured because, based on equation (9), no consideration is given to the directivity corresponding to the J.sub.z component of a probe antenna used in the measurement of V(R). In addition, only two components of the wavesource current vectors can be visualized.